Introduction
The invention relates to an adjustment mechanism for furniture, and in particular furniture having at least two parts whose relative position may be adjusted and locked. One example is a swivel chair in which a seat and a backrest are movable with respect to each other, and in which the seat may in turn be movable with respect to a fixed support on a spindle. Another example is a "scissors" height adjustment mechanism for a collapsible table, or a pivotable height adjustment mechanism for computer furniture.
Prior Art
At present, such adjustment mechanisms generally comprise a locking device which is usually a friction clutch. European Patent Specification No. 0,394,784 (Lineager) and PCT Patent Specification No. 86/00508 (Volkle) describe adjustment mechanisms for swivel chairs. In both of these mechanisms, there is a biasing spring which acts between a friction clutch and a cam at the end of a handle protruding from the mechanism. To disengage the clutch, the cam is rotated using the handle to release pressure of the spring on the friction clutch to allow movement of the various parts. Problems with this arrangement include the fact that the cam and handle arrangement is bulky, for example, in EP 0,394,784 the spring and cam arrangement protrudes outside of a channel which encloses the clutch and other of the adjustment mechanism. Because they are bulky, it appears they would be relatively expensive to manufacture. Another significant disadvantage is that pressure from the spring acting on the friction clutch also acts on the cam and handle which are used for operation of the friction clutch. These stresses are in turn transmitted from the cam and handle to the housing for the adjustment mechanism. Accordingly, there would be significant wear on the mechanisms because of the effects of the stress.
European Patent Specification No. 0,045,925 (Steifensand) also discloses an adjustment mechanism for a chair. In this case a threaded spindle is used to clamp the leaves of the friction clutch together. Again, stresses would be transmitted from the friction clutch, through the spindle and into various other parts of the mechanism.
British Patent Specification No. GB 2,193,884 (Chair Mechanisms Limited) describes an adjustment mechanism for a chair which is less bulky than those referred to above. In this case, the friction clutch is clamped in the engaged position by a transversely mounted clamp which is acted upon by a torsion spring. However, the significant disadvantage remains that spring pressure is transmitted through the housing of the mechanism, again leading to reliability problems and more costly construction requirements.
Another disadvantage of presently available adjustment mechanisms for furniture generally is that if there is more than one locking device for relatively movable parts, each must have an associated clamp, cam or threaded spindle arrangement for operation of the clutch. This leads to a requirement for a large number of parts and for a large size and costly manufacture.